The sd driver provides support for a SCSI disk. It
allows the disk to be divided up into a set of pseudo devices called
partitions. In general the interfaces are similar
to those described by
wd(4).

SCSI devices have a fairly high level interface to the system and talk to it via
a SCSI host adapter (e.g.,
ahc(4)). A SCSI adapter
must also be separately configured into the system before a SCSI disk can be
configured.

When the SCSI adapter is probed during boot, the SCSI bus is scanned for
devices. Any devices found which answer as
‘Direct’ type devices will be
attached to the sd driver.

On many systems
disklabel(8) is
used to partition the drive into filesystems. On some systems the
OpenBSD portion of the disk resides within a native
partition, and another program is used to create the
OpenBSD portion.

For example, the i386 port uses
fdisk(8) to partition
the disk into a BIOS level partition. This allows sharing the disk with other
operating systems.

Read, from the kernel, the in-core copy of the disklabel
for the drive. This may be a fictitious disklabel if the drive has never
been initialized, in which case it will contain information read from the
SCSI inquiry commands.

If a removable device is attached to the sd driver,
then the act of changing the media will invalidate the disklabel and
information held within the kernel. To avoid corruption, all access to the
device will be discarded until there are no more open file descriptors
referencing the device. During this period, all new open attempts will be
rejected. When no more open file descriptors reference the device, the first
next open will load a new set of parameters (including disklabel) for the
drive.